Mobile telephone systems have evolved over time from the simple radiotelephone systems of the 1970s to complex multiplexed digital systems which are now becoming standard. Such changes represent a need to handle growing numbers of cellular phone users as well as improvements in technology and reductions in cost of electronic mobile phone equipment. But, as technology and use of mobile phones increases, the frequency spectrum available for electronic wireless communication remains fixed and so necessitates increasingly efficient use of what frequency bandwidth is available for mobile phone systems.
In a cellular phone system where frequencies within the allotted frequency band are reused in cells not physically adjacent to each other, capacity to handle additional calls can be increased by simply adding more cells in a geographic area. But, technical problems and costs associated with very dense cell population make solutions based on more efficient use of bandwidth a more cost-effective and practical option. The CDMA wireless phone system is one example of a solution that allows multiple cellular phone users to share the same frequency spectrum, and uses a generated noise carrier with a different and essentially orthogonal instance of the noise carrier assigned to each mobile unit within a cell. The base station receiver in a CDMA station correlates the received signal from a mobile unit with the desired noise carrier, extracting the transmitted digital signal with a sufficient signal-to-noise ratio to achieve a satisfactory data error rate.
But, because the base stations in a system such as cDMA must be synchronized with surrounding base stations to handle handoff of mobile phones between cells and for other functions, a time reference must be provided to each base station. This is commonly provided via Global Positioning System (GPS) receivers which comprise a part of each base station. GPS satellites each provide radio signals that are synchronized and usable by GPS receivers not only to derive one's physical position relative to the satellites but also to derive a very accurate time reference.
But, because the GPS receiver antennas of cellular phone equipment are often placed high relative to surrounding terrain, they are subject to lightning damage in addition to physical damage from rough handling or other damage. CDMA base stations which lose contact with GPS satellites should ideally continue to operate during this holdover period until contact can be reestablished, whether through repair of damaged equipment, or other changed circumstances. A crystal oscillator may provide a time reference during this holdover period, as long as the oscillator is stable enough to keep the base station sufficiently synchronized with other base stations.
Crystal oscillators in base stations used to provide a holdover time reference are often tested before placement in the field by monitoring timing stability during a pre-deployment test that typically lasts two days or less. Unfortunately, some crystals do not become stable in frequency or settle in until several weeks of operation have passed, and once settled in may prove to be too inaccurate or unstable to provide an adequate time reference during a holdover period. As crystals age they also tend to drift in frequency, and the frequency change again becomes more rapid as crystals begin to fail. A method of testing crystals after a longer period of operation is needed, as is a method of compensating for frequency change in a crystal over time.